How to take self portraits like this with a tripod?

Since the person in this photo is in a bathtub (or pool) it is pretty safe to say that a tripod was not used.

You need some way to get the camera to point down without showing the tripod legs. One way yo do it would be to suspend the tripod from a tree and point the camera straight down. You could do have the legs all in one bundle, which will allow the tripod and camera to swivel, or you could open the legs and brace each one individually. You could also get a Gorillapod and wrap it around a tree limb or other suspension support:

http://www.adorama.com/TPGPSLRZBC.html

If you have a ball head, or can take it off your tripod, you can get a 3/8-16 stud/screw adapter bolt (http://www.fmwfasteners.com/products/Hanger-Bolts-... and thread it in to wherever you want to hang the camera. Then screw the ball head onto the 3/8-16 stud and mount your camera.

Or you can get a tripod that works like a boom, just make sure the legs are weighted so it does not tip over:

http://www.adorama.com/VGALTAP263AT.html

Well, I actually made a "boom" that attaches to my tripod and supports the camera in a horizontal position to the tripod for vertical shots like that. mostly for product or still life work. You'd be amazed at how welding is a handy skill to have. Anyway, if a dummy like me could come up with a rig like that, I'm sure there's something available out there that does the same thing. I'd hate to know how much it costs though. I made mine for less than $100.

That photo description has a link to an article which itself has an entire series of 19 "behind the scenes" shots which pretty clearly show how the photo was made.

Could he have done it by himself? Sure. Wireless remote or very long self-timer. Read the article and see the background photos for details. These kind of shots are not horribly difficult to set up, they simply require some ingenuity and/or "outside the box" thinking. And of course, Photoshop composites can always give you a bit more of an edge and/or cover-up side-effects of the setup.

http://www.ocregister.com/articles/alex-382139-pho...

(Select the small image at the top of the article to open the photo gallery).

This image immediately made me think of Brooke Shaden who has an extensive self-portrait collection of herself (and other models) in various "fantasy" scenes. Much of her work arguably relies on Photoshop (I'm not saying that to be a dig at her technique -- that's just her style). She is an instructor and would be happy to sell you all sorts of videos on her techniques, haha, but she also has some free videos on youtube --

http://www.youtube.com/user/brookeshaden

Lay the tripod on a table, head jutting out at the edge with all the legs pulled out. You can get creative here and push the table to the edge of the window of the second floor bedroom and have the camera jut out so you can pose down on the garden with more area to cover. Pile a ton of your school books on top of the legs making sure to pin the tripod down. IF unsure, put the sofa or refrigerator on top of the tripod. Attach the camera to the tripod. Set to timer mode, press button, run to position and pose.